It is known to use a position detecting system as an input device for a computer aided design system (CAD), a computer, or a portable telephone. These position detecting systems include a position detecting apparatus in which a signal is transmitted and received by means of electromagnetic coupling or electrostatic coupling between a position pointer and a tablet. A position pointed to by the position pointer is detected by the tablet based on the transmitted/received signal. Examples of such position detecting apparatuses may be found, for example, in patent documents listed below:
Patent Document 1: Japanese Unexamined Patent Application Publication No. 3-189716
Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-175572
Patent Document 3: U.S. Pat. No. 5,679,930
Patent Document 4: Japanese Unexamined Patent Application Publication No. 3-189717
Patent Document 5: Japanese Unexamined Patent Application Publication No. 7-239746
Patent Document 6: Japanese Unexamined Patent Application Publication No. 7-219698
Patent Document 7: Japanese Unexamined Patent Application Publication No. 7-234752
Patent Document 8: Japanese Unexamined Patent Application Publication No. 7-261909
Patent Document 9: Japanese Unexamined Patent Application Publication No. 10-133803
Patent Document 10: Japanese Unexamined Patent Application Publication No. 63-70326
In position detecting apparatuses disclosed in Patent Documents 1 to 3, a position pointer has a resonant circuit. A characteristic of the resonant circuit is controlled in synchronization with an electromagnetic wave transmitted from a tablet, depending on a status of a switch of the position pointer or depending on data indicating a pointer pressure against the tablet or the like transmitted from the position pointer. In those techniques disclosed in Patent Documents 1 to 3, the position pointer does not have a battery. The electric power with which the position pointer operates is obtained by rectifying a signal induced by the electromagnetic wave transmitted from the tablet.
When a position detecting apparatus of this type is used with a battery-powered portable personal computer, the electric, power of the position detecting apparatus is acquired from an electromagnetic wave transmitted from the portable personal computer. In some cases, a position detecting apparatus is disposed on a personal computer to realize a capability of inputting data via a liquid crystal screen using a pen. In these position detecting apparatuses, in order for a position pointer to acquire sufficient electric power, a tablet has to transmit an electromagnetic wave with sufficiently large power, which causes a reduction in life of a battery of a portable personal computer.
To solve the above-described problem, Patent Document 4 disclose's a technique to reduce electric power consumed by a tablet by disposing a battery in a position pointer whereby the position pointer operates with electric power supplied from the battery. It is also known to dispose a battery and an oscillator in a position pointer whereby a high-frequency signal generated by the oscillator is radiated in the form of an electromagnetic wave from the position pointer, and a tablet detects a position pointed to by the position pointer by detecting the electromagnetic wave.
Patent Document 5 discloses a technique using electrostatic coupling. In this technique, a battery and an oscillator are disposed in a position pointer, and a high-frequency electric field generated by the position pointer is detected by a tablet.
In the technique in which a battery is disposed in a position pointer, a power switch disposed on the position pointer is turned off when the position pointer is not used. However, a manual operation to turn on/off the power switch is troublesome for a user.
In some position pointers, there is no power switch, and power is automatically turned off to reduce useless consumption of electric power of a battery when no operation (pressing of a button, pressing the position pointer against a tablet, or the like) is performed with the position pointer in a predetermined period. However, the period is set to be long enough to allow the position pointer to be used in a convenient way, and thus wasting of electric power of the battery occurs.